Moving pictures are supposed to move, but the most moving ones in “The Salt Of the Earth” are stills. This exemplary documentary celebrates the work of the Brazilian photojournalist Sebastião Salgado; it was directed by Wim Wenders and the subject’s son, Juliano Ribeiro Salgado. You may know the elder Salgado’s work from his iconic images—the overused adjective has never been more apt—of a Brazilian gold mine, with tens of thousands of manual laborers digging the soil simultaneously in a single, immense hole. (Some of those images provide the documentary’s opening sequence.) Mr. Salgado has chronicled anguish,...